Before 1998, only 6 percent of no-hitters were perfect games, but from 1998 to 2003, 20 percent were, and since then 27 percent have been.

From 1900 through 1980, baseball witnessed only seven perfect games, including two in the dead-ball era and three during the glory days for pitchers in the mid-1960s. But in the 30 seasons beginning with 1981, nine pitchers have achieved perfection. And, oddly, regular no-hitters have decreased in frequency while becoming more erratic in their appearance.

“There is probably a fair amount of chance involved” in the jump in perfect games, said Rob Neyer, a baseball columnist with ESPN. He says the rise of free-swingers helps because strikeouts mean fewer balls in play and thus fewer possible hits and errors — although he noted that fewer balls in play should also mean more no-hitters. Improved fielding has probably helped as well, he said.

In the 20 years before Babe Ruth and the live ball era of 1920, no-hitters were far more common, with pitchers hurling 48 of them. In the two decades that followed — the most explosive offensive period before the steroids era — there were just 16 no-hitters, one of which was a perfect game. Baseball found an equilibrium in the 1940s and ’50s and that span yielded 30 no-hitters.

Not surprisingly, the swinging (and missing) ’60s produced an astonishing 30 no-hitters, along with three perfect games, but even after baseball lowered the pitching mound, pitchers churned out 31 no-hitters in the 1970s. In other words, from 1960-1979, baseball averaged more than three no-hitters per season yet only one perfect game about every seven years. Since then, the pattern has shifted: There have been 48 no-hitters over the past 30 years, meaning it now takes two seasons to produce three no-hitters. In the last decade, there were only 13 — none in 2000 and only one (a perfect game) from June 11, 2003, through Sept. 6, 2006.

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Oakland’s Dallas Braden accomplished the feat against the same team on Sunday.Credit
Carlos Avila Gonzalez/San Francisco Chronicle, via Associated Press

That decline might make sense considering that the strike zone, the ballparks, the ball and steroids all conspired to boost offense, yet there have been 10 perfect games in that span, meaning they are now coming along every three seasons on average instead of every seven. In fact, perfect games before this year were fairly evenly spaced out, appearing in 1981, 1984, 1988, 1991, 1994, 1998, 1999, 2004 and 2009.

As for this year’s big events, it makes sense that Ubaldo Jimenez would pitch the no-hitter and Braden the perfect game — 25 of 35 no-hitters since 1988, including Jimenez’s, have been pitched by right-handers while six of the eight pitchers to retire all 27 hitters they faced, including Braden, have been southpaws.

The only question now is whether Braden can live up to his place in baseball history. While pitchers of all stripes have pitched no-hitters — Justin Verlander pitched one in the last decade but so did Anibal Sanchez and Bud Smith — the perfect-game club is more selective.

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Credit
The New York Times

Of the 17 members, five are Hall of Famers (Cy Young, Addie Joss, Jim Bunning, Sandy Koufax and Catfish Hunter) while Charlie Robertson (who never had a winning season) is forgotten and Don Larsen is remembered only for his one magical day in the 1956 World Series. And while the 1980s had one undistinguished pitcher (Len Barker) and two solid but unremarkable ones (Mike Witt and Tom Browning) pitch perfect games, that has changed since 1990.

For in the last two decades, every perfect game has come from a pitcher with an impressive résumé.

Randy Johnson is headed for a first-ballot election to the Hall of Fame. (He also pitched a no-hitter.) Dennis Martinez won 245 games and an earned run average title; David Wells won 239 games and had a 10-5 record with a 3.17 E.R.A. in the postseason; David Cone had two 20-win seasons, one Cy Young Award, two strikeout crowns and five All-Star Game appearances.

Even Kenny Rogers, who is despised by many New York fans, won 219 games. Last year’s perfect-game pitcher, Mark Buehrle, has a chance to end up in that circle, having already racked up 137 wins and four All-Star Game appearances at age 31.

So while we await the next perfect game, which might be here sooner than we think (pay attention to those lefties), keep an eye on Dallas Braden: given that his E.R.A., walks, hits and home runs per inning pitched have steadily declined over the past four years, this perfect game may not be an aberration but the start of something big.

A version of this article appears in print on May 14, 2010, on Page B11 of the New York edition with the headline: Changeup In Perception Of Perfect. Order Reprints|Today's Paper|Subscribe